Linear alkylbenzene sulfonate (LAS) and primary alkyl sulfate (PAS) are widely used as anionic surfactants in commerce. They find special application for use in light and heavy duty liquid and powdered detergents. A potential disadvantage of LAS and PAS, however, is that under hard water conditions, i.e., calcium levels greater than about 150 parts per million, these anionic materials can interact with cationic water hardness ions, such as calcium, thereby becoming inactivated through precipitation. While this is a problem common to anionic surfactants, LAS and PAS are especially sensitive to water hardness ions.
Although not wishing to be bound by any theory, the literature indicates that the aforementioned interaction can best be understood by considering the micellar structure of these anionic surfactants such as , for example, LAS. Repulsive forces between negative charges in the sulfonate group lead to a higher critical micelle concentration (CMC) than, for example, with a nonionic surfactant. CMC is the surfactant concentration at which micellar formation begins. Stated otherwise, the negative charge of LAS retards micellar formation and shifts the equilibrium towards the monomer. A relatively high monomer concentration in solution results thereby; this is significant because precipitation between calcium ion and LAS occurs only with the monomer.
It has been found that powdered laundry detergent compositions containing secondary alkyl sulfate as an anionic surfactant component can be used in place of powdered laundry detergent compositions containing linear alkylbenzene sulfonates and/or primary alkyl sulfates with the advantages being increased tolerance for water hardness ions, good detergency properties, increased dissolution rates, higher active matter contents, lower water contents, and improved color of raw materials.